Mat Lee, from Derbyshire Fire Service, told Nottingham Crown Court the blaze at a house in Allenton, Derby, last May started in the hallway in front of the door and quickly spread up the stairs to the three bedrooms.

Mick and Mairead Philpott, and a third defendant, Paul Mosley, are on trial accused of the manslaughter of the Philpotts' six children, who died following the blaze at their house on May 11 last year. They deny the charges.

Today Mr Lee told the jurors: "The most likely cause of the fire was in my opinion the deliberate emission of petrol vapour by a naked flame or spark on the ground floor by a person."

Earlier the court heard covert recordings made in a police van taking the Philpotts to the magistrates court in Derby following their arrest 18 days after the fire.

Philpott is alleged to have whispered to his wife during the journey on May 31: "You definitely sticking to the story? I didn't mean to do it. On my life." But the comment was disputed by defence barristers.

Philpott's barrister Anthony Orchard QC, argued that there was too much background noise for the comment to be heard clearly and disputed that his client made the comment "I didn't mean to do it".

Nottingham Crown Court has heard the Philpotts were being monitored 24 hours a day by Serious and Organised Crime detectives.

In the van, Philpott is heard asking his wife: "Are we sticking to the story?"

Mairead replies: "Hmmm". Mick whispers: "Good."

He then asks her: "Do you know something? Have they got any evidence on you? They've got nothing on me, nothing."

Mrs Philpott is heard to say: "There's a fingerprint on the window."

He replies: "That's it. A few discrepancies, that's all it is. One with Shakey (Paul Mosley) about I said I was going out for a drive at night. I can't remember saying that, Vicky said you were stoned and I said we didn't smoke."

He is then heard to say to his wife: "I love you so much. Look at me. We will walk free and prove our innocence, right? And then we'll sort them out, promise you."

Mrs Philpott replies: "Yeah, like I said, we can't even bury the kids."

Philpott then says: "Mick, my brief, said that for the judge to do what he's doing he's got to have some big balls because there's no evidence, they've got nothing, there's no, no petrol on me, on me trousers, no glass, you know the fireman said he put the window through.

A few minutes later he asks his wife: "You definitely sticking with the story? I didn't mean to do it, on my life".

During the 10-minute conversation, Philpott is also heard to say "they tried to blame me". His wife says: "I know."

She adds: "They said 'Did you pour the petrol there?', cause they said summat about erm... expert thingy."

Philpott replies: "Load of bu*****t."

His wife is heard to say: "That's why the thingy said just no comment."

He then says: "Don't you worry, we'll walk through it, I promise you that. Unless you want me to blab about it?"

His wife replies: "No." Philpott then says: "It's your decision."

This afternoon the court heard how the fire started in the hallway and spread up the stairs, helped by an open window at the top of the landing.

Mr Lee, who arrived at the scene hours after the flames had been extinguished, told the court his investigations had led him to conclude that the fire was set in the hallway.

He said the blaze quickly spread into the living room, melting polystyrene ceiling tiles which dropped onto the carpet, setting it and a curtain alight. The fire also spread up the carpeted stairs to the bedrooms where the six children were asleep in their beds, he said.

Giving evidence today, Mr Lee told the court that most of the UPVC front door, which had been locked during the fire, had been burnt away by the intense heat.

He said smoke patterns found during expert investigations suggested the flames came from inside the property rather than the outside.

No evidence of ignitable materials were found on the front step or on the door mat, Mr Lee told the court.

When asked by Richard Latham QC how and where the fire was most likely to have started, Mr Lee told the jury: "The fire started directly behind the front door in the hallway. Petrol vapour was ignited by a spark or more than likely a naked flame.

"The front door began to decompose. The fire developed due to the involvement of the flames and fumes from the UPVC door."

He said the fire moved up the staircase assisted by the carpet cladding and an open window at the top of the landing.

Blackened walls upstairs showed the upstairs doors were open, allowing the heat and smoke to spread into the bedrooms, he said.

Earlier clips from the 999 call made by the distressed parents in the early hours of May 11 were replayed to the jury.

During the clips, which were also heard earlier in the trial, Philpott and his wife can be heard crying and pleading for help from the emergency services and telling the operator that the six children are trapped in the upstairs bedrooms.

Jade, 10, and brothers John, nine, Jack, eight, Jesse, six and Jayden, five, all died following the blaze. Their brother Duwayne, 13, was taken to Derby Royal Hospital and transferred to Birmingham Children's Hospital but died three days later.

All six died from smoke inhalation, the court has heard.

The prosecution claim their deaths were the result of a "plan" to frame Philpott's former mistress Lisa Willis that went wrong.

The court has heard how Miss Willis left the home she had shared with the couple and their six children in February, taking her five children with her.

The jury has been told Philpott and Miss Willis were due to face each other at a court hearing over access to the five children, four of which he had fathered, on the day of the blaze.